The demand for color-coated steel sheets for use in building materials, household appliances, cars, etc, is increasing. In particular, in domestic and foreign cold-rolled product manufacturers which produce hot-dip galvanized steel sheets and electro-galvanized steel sheets and in surface treatment companies which treat other steel materials, products are produced by performing, as final post-treatment, chemical treatment processes such as chromate treatment and phosphate treatment.
Herein, the chromate treatment refers to covering a steel sheet with an anti-rust coating layer by immersing the steel sheet in a solution containing chromate or dichromate as a main component. This chromate treatment is an inexpensive treatment process that imparts excellent corrosion resistance and coating adhesion to the steel sheet.
However, because chromium is one of representative toxic pollutants and causes serious damage to workers and the environment, regulations for restricting the use of such toxic pollutants have been enacted in advanced countries, and such regulations have been enforced worldwide.
In addition, in the case of chromate-treated steel sheets, hexavalent chromium (Cr+6) is generated in wastewater during the process, and thus treatment of this wastewater is highly expensive and time-consuming. Moreover, because it is expected that the use of chromate treatment will be prohibited in the near future through international regulations on the use of chromium, studies on chromate treatment are now no longer conducted, and studies on the development of materials capable of replacing chromium have been actively conducted worldwide.
Regarding studies conducted to date, studies have been conducted to develop a solution of 100% trivalent chromium by reducing the amount of hexavalent chromium because hexavalent chromium is more toxic than trivalent chromium. However, these studies do not aim at removal of chromium, and the 100% trivalent chromium solution does not have any merit to replace an existing chromate solution in view of corrosion resistance and price.
Thus, in recent years, efforts have been made to use water-soluble type ceramic coatings to provide both excellent hardness and processability. However, ceramic coatings have disadvantages in that they cannot be processed or require a high treatment temperature and a very long treatment time, which lead to low yield and productivity. In addition, when ceramic coatings are used, there is a problem in that the ceramic coatings can be applied only to sheets.
Prior art documents related to the present invention include Korean Laid-Open Patent Publication No. 10-2005-0073790 (published on Jul. 18, 2005) which discloses a coating for a color-coated steel sheet, which shows various patterns, and a method for manufacturing a color-coated steel sheet using the same.